In order to attain higher fuel economy in automobiles, transport airplanes, etc., it is desired to reduce an empty weight of an automobile or transport airplane. A technique of using a high strength steel sheet and reducing a thickness thereof is effective for the weight reduction. In particular, automobiles are required to ensure collision safety. For example, structural components such as a pillar, and reinforcing components such as a bumper and an impact beam, are required to further increase the strength thereof. However, in general, as the strength of a steel sheet is increased, ductility will be deteriorated, resulting in poor workability. Therefore, there is a need for a steel sheet capable of satisfying both high strength and high ductility.
As a steel sheet having both high strength and high ductility, great interest has been shown in a TRIP (Transformation Induced Plasticity) type steel sheet. As one example thereof, there has been known a TBF steel sheet which comprises: bainitic ferrite as its parent phase; and retained austenite (hereinafter occasionally denoted as “retained γ”) (see, for example, the following Non-Patent Document 1). In the TBF steel sheet, high strength is obtained based on hard bainitic ferrite, and excellent ductility is obtained based on fine retained γ existing in boundaries of the bainitic ferrite.
Meanwhile, a steel sheet for use in automobiles and transport airplanes is also required to be resistant to the occurrence of delayed fracture due to hydrogen embrittlement (hereinafter referred to occasionally as “hydrogen embrittlement resistance”). The delayed fracture means a phenomenon that hydrogen generated in a corrosive environment or hydrogen in the atmosphere diffuses into defective areas, such as dislocations, holes and grain boundaries, in the steel sheet, to embrittle the defective areas and cause deterioration in ductility and rigidity of the steel sheet, and thereby fracture will occur under a condition that static stress causing no plastic deformation is applied to the steel sheet.
As a technique for improving hydrogen embrittlement resistance of the TBF steel sheet comprising retained γ, the following Patent Documents 1 to 5 have been known. Among them, the Patent Document 1 discloses a technique for improving hydrogen embrittlement resistance of a high-strength thin steel sheet which comprises a main phase consisting of bainite and bainitic ferrite, and a second phase consisting of austenite, with the remainder being ferrite and/or martensite, and has a tensile strength of 800 MPa or more. This Document includes a description mentioned that, in order to improve the hydrogen embrittlement resistance, the strength and composition of the steel sheet are adjusted to control a deposit serving as a hydrogen trap site, and the composition of the steel sheet is adjusted to reduce a rate of hydrogen penetration into the steel sheet.
The Patent Documents 2 to 5 disclose techniques which were previously proposed by the applicant of this application. Metallographic structures of steel sheets disclosed in each of these Documents comprise 1 area % or more of retained γ, and 80 area % or more of a total of bainitic ferrite and martensite. These Documents include a description mentioned that the parent phase of the steel sheet may be formed in a two-phase structure of bainitic ferrite and martensite to reduce origins of intergranular fracture, and retained γ is formed in a lath-like configuration to enhance a hydrogen trapping capability to allow hydrogen to become harmless so as to improve the hydrogen embrittlement resistance.
The steel sheet for automobiles and transport airplanes is required to satisfy both high strength and high ductility, as mentioned above. Particularly as for strength, it has recently been required to satisfy a tensile strength of 1180 MPa or more. However, if the tensile strength is increased to 1180 MPa or more, the delayed fracture due to hydrogen embrittlement is more likely to occur. Therefore, in the Patent Documents 2 to 4, the applicant disclosed and proposed a technique intended for a high strength steel sheet having a tensile strength of 1180 MPa or more and designed to improve the hydrogen embrittlement resistance, and obtained a certain level of effect. However, there is a need for further improving the hydrogen embrittlement resistance.